Medium format (big) lens, effect on hotspot at DVinfo.net

Les Dit · March 10th, 2005, 09:10 PM

I had some time today to try an experiment. I wanted to see if using a larger format lens might reduce the hotspot problem, which is encountered when using a groundglass. my previous tests used a 50 mm 35 format still camera lens. The hotspot was fairly pronounced.
Today I rigged up a quick test using a static groundglass with a bigger lens. A much bigger lens. This lens has a front diameter of 3 in. and the rear element is 4 in. in diameter. Its original application was for aerial photography for the military. The image plane produced by the lens is approximately twice as big as my 36 mm groundglass.

This configuration still produces a significant hotspot at all apertures of the lens. I'm concluding that the hotspot has more to do with that diffusion properties of the groundglass at this point.

Does anybody have a high-resolution photograph shot of a Fresnel screen without the screen in motion? I'm still a doubting Thomas as far as using one of those with 720 P. video. I have yet to play with one.

-Les

Leo Mandy · March 10th, 2005, 09:36 PM

Les,

If making the back-lens bigger didn't fix the problem, what do you think will?

Les Dit · March 11th, 2005, 12:15 AM

Mandy,
I'm not sure what you mean by "back lens ". To erase any ambiguity this lens I was testing was between the action and the groundglass. I was using no condenser lens between the diopters on the video camera and the groundglass.
I'm still looking for solutions that maintain sharpness and allow the maximum use of that groundglass/image plane area.
I actually tried a fiber-optic image transfer plate instead of a groundglass, it seemed to help but would be too heavy to move around (orbital pattern).

-Les

<<<-- Originally posted by Mandy Leo : Les,

If making the back-lens bigger didn't fix the problem, what do you think will? -->>>

Brett Erskine · March 11th, 2005, 01:58 AM

Beattie focusing screens are well known for their sharpness, contrast and brightness and have a extremely fine fresnel lens built into the GG to do the job that the condenser normally would do. I picked up a custom made focusing screen from a little know but highly respected company by the name of Maxwell Precision Optics. They are said to be a tad better than even the Beattie screens. Normally I would say dont use a fresnel because a normal lens in theory will produce better images than a fresnel lens but in this case the fresnel is SO sharp and microscopic that it worked perfectly for me. No hotspot and no color abberations problems. I hear you about the HD issue. I'm sure you heard me mention this before but just in case - I'm using medium format lenses and a larger focusing screen for my adapter. The extra sized GG makes for a crisper image. I cant wait to see how much better it looks when I put it in motion.

Daves Spi · March 11th, 2005, 02:11 AM

Look at this:
http://web.datriware.com/gfx_photos/articles/p881.jpg
http://web.datriware.com/gfx_photos/articles/p891.jpg
Its full 36mm image, and no special hotspot is visible. I shot it with normal 35mm lens... Its static. I build it just for testing from Nikon focusing screen for $10. Its not beattie.

But I bought Maxwell's lens at eBay for GBP 60, so I can't wait any more to test it... Hope for super image... ;-)

Leo Mandy · March 11th, 2005, 06:57 AM

What I meant, and sorry for not being clear, is the back of the lens - the part that faces the GG. If it is bigger, doesn't that help with having less of a hotspot? But from what you are saying, it does not help.

Les Dit · March 11th, 2005, 02:10 PM

I guess I'll get a Maxwell screen to try. What worries me is that these were never designed as taking lenses. I don't think there has ever been a commercial camera that promotes a Fresnel as its taking lens. They are not known for their resolving power. Those steps just don't lend themselves to good MTF properties. We are moving away from WebCam resolution these days. :-)
-Les

Aaron Shaw · March 11th, 2005, 03:11 PM

Agreed Les. It's hard to tell just how much the fresnel is affecting the MTF. It would be interesting to find out! That's the only reason I haven't yet gone with a focusing screen.

Leo Mandy · March 11th, 2005, 03:23 PM

What does MTF stand for?

Aaron Shaw · March 11th, 2005, 03:44 PM

MTF stands for "Modulation Transfer Function"

It's basically a way of determining lens contrast. By lens contrast I mean the ability of a lens to distinguish clearly between different boundaries in luminance. It can also be thought of, simply, as the ability of the lens to tell the difference between fine lines and close tonal values.

Leo Mandy · March 11th, 2005, 03:50 PM

Is there a decent price point for Fresnels or Beatties? I am looking into getting one myself, but don't know where to start for a good price, but decent quality.

Rokta Bija · March 18th, 2005, 08:52 PM

Les,

When you used the medium format lens on your static test, how did the image compare to the 35mm lens as far as sharpness and grain? Was your GG large enough to be able to use the full size of the medium format image?

Les Dit · March 18th, 2005, 11:36 PM

Just some clarification: the lens I was using was originally for aerial photography. I believe the sharpness and grain imaged was identical to a 35mm still camera lens. Unfortunately , the hotspot was also pretty much the same. Even though the rear lens element was 4 in. in diameter!
<<<-- Originally posted by Rokta Bija : Les,

When you used the medium format lens on your static test, how did the image compare to the 35mm lens as far as sharpness and grain? Was your GG large enough to be able to use the full size of the medium format image? -->>>

Craig Bellaire · March 30th, 2005, 12:39 PM

can you e-mail me... thanks craigbellaire@hotmail.com

March 10th, 2005, 09:10 PM	#1
Les Dit Major Player Join Date: Oct 2003 Location: Southern Cal-ee-for-Ni-ya Posts: 608	Medium format (big) lens, effect on hotspot I had some time today to try an experiment. I wanted to see if using a larger format lens might reduce the hotspot problem, which is encountered when using a groundglass. my previous tests used a 50 mm 35 format still camera lens. The hotspot was fairly pronounced. Today I rigged up a quick test using a static groundglass with a bigger lens. A much bigger lens. This lens has a front diameter of 3 in. and the rear element is 4 in. in diameter. Its original application was for aerial photography for the military. The image plane produced by the lens is approximately twice as big as my 36 mm groundglass. This configuration still produces a significant hotspot at all apertures of the lens. I'm concluding that the hotspot has more to do with that diffusion properties of the groundglass at this point. Does anybody have a high-resolution photograph shot of a Fresnel screen without the screen in motion? I'm still a doubting Thomas as far as using one of those with 720 P. video. I have yet to play with one. -Les

March 11th, 2005, 02:11 AM	#5
Daves Spi Regular Crew Join Date: Feb 2005 Location: Czech republic, Prague Posts: 159	Re: Medium format (big) lens, effect on hotspot Look at this: http://web.datriware.com/gfx_photos/articles/p881.jpg http://web.datriware.com/gfx_photos/articles/p891.jpg Its full 36mm image, and no special hotspot is visible. I shot it with normal 35mm lens... Its static. I build it just for testing from Nikon focusing screen for $10. Its not beattie. But I bought Maxwell's lens at eBay for GBP 60, so I can't wait any more to test it... Hope for super image... ;-) __________________ Daves At the beginning there was an idea, then the ambition came and the idea became to be a dream... The Satisfied Dream => http://film.datriware.com

March 30th, 2005, 12:39 PM	#14
Craig Bellaire Major Player Join Date: Aug 2004 Location: Rochester, NY Posts: 422	Rokta Bija can you e-mail me... thanks craigbellaire@hotmail.com __________________ Craig Bellaire www.VerticalVideoSolutions.com

March 10th, 2005, 09:36 PM	#2
Leo Mandy Major Player Join Date: Dec 2004 Posts: 749	Les, If making the back-lens bigger didn't fix the problem, what do you think will?

March 11th, 2005, 12:15 AM	#3
Les Dit Major Player Join Date: Oct 2003 Location: Southern Cal-ee-for-Ni-ya Posts: 608	Mandy, I'm not sure what you mean by "back lens ". To erase any ambiguity this lens I was testing was between the action and the groundglass. I was using no condenser lens between the diopters on the video camera and the groundglass. I'm still looking for solutions that maintain sharpness and allow the maximum use of that groundglass/image plane area. I actually tried a fiber-optic image transfer plate instead of a groundglass, it seemed to help but would be too heavy to move around (orbital pattern). -Les <<<-- Originally posted by Mandy Leo : Les, If making the back-lens bigger didn't fix the problem, what do you think will? -->>>

March 11th, 2005, 01:58 AM	#4
Brett Erskine Major Player Join Date: Dec 2003 Location: Anaheim, CA Posts: 445	Beattie focusing screens are well known for their sharpness, contrast and brightness and have a extremely fine fresnel lens built into the GG to do the job that the condenser normally would do. I picked up a custom made focusing screen from a little know but highly respected company by the name of Maxwell Precision Optics. They are said to be a tad better than even the Beattie screens. Normally I would say dont use a fresnel because a normal lens in theory will produce better images than a fresnel lens but in this case the fresnel is SO sharp and microscopic that it worked perfectly for me. No hotspot and no color abberations problems. I hear you about the HD issue. I'm sure you heard me mention this before but just in case - I'm using medium format lenses and a larger focusing screen for my adapter. The extra sized GG makes for a crisper image. I cant wait to see how much better it looks when I put it in motion.

March 11th, 2005, 06:57 AM	#6
Leo Mandy Major Player Join Date: Dec 2004 Posts: 749	What I meant, and sorry for not being clear, is the back of the lens - the part that faces the GG. If it is bigger, doesn't that help with having less of a hotspot? But from what you are saying, it does not help.

March 11th, 2005, 02:10 PM	#7
Les Dit Major Player Join Date: Oct 2003 Location: Southern Cal-ee-for-Ni-ya Posts: 608	I guess I'll get a Maxwell screen to try. What worries me is that these were never designed as taking lenses. I don't think there has ever been a commercial camera that promotes a Fresnel as its taking lens. They are not known for their resolving power. Those steps just don't lend themselves to good MTF properties. We are moving away from WebCam resolution these days. :-) -Les

March 11th, 2005, 03:11 PM	#8
Aaron Shaw Major Player Join Date: Jul 2004 Posts: 587	Agreed Les. It's hard to tell just how much the fresnel is affecting the MTF. It would be interesting to find out! That's the only reason I haven't yet gone with a focusing screen.

March 11th, 2005, 03:23 PM	#9
Leo Mandy Major Player Join Date: Dec 2004 Posts: 749	What does MTF stand for?

March 11th, 2005, 03:44 PM	#10
Aaron Shaw Major Player Join Date: Jul 2004 Posts: 587	MTF stands for "Modulation Transfer Function" It's basically a way of determining lens contrast. By lens contrast I mean the ability of a lens to distinguish clearly between different boundaries in luminance. It can also be thought of, simply, as the ability of the lens to tell the difference between fine lines and close tonal values.

March 11th, 2005, 03:50 PM	#11
Leo Mandy Major Player Join Date: Dec 2004 Posts: 749	Is there a decent price point for Fresnels or Beatties? I am looking into getting one myself, but don't know where to start for a good price, but decent quality.

March 18th, 2005, 08:52 PM	#12
Rokta Bija Regular Crew Join Date: May 2004 Posts: 61	Les, When you used the medium format lens on your static test, how did the image compare to the 35mm lens as far as sharpness and grain? Was your GG large enough to be able to use the full size of the medium format image?

March 18th, 2005, 11:36 PM	#13
Les Dit Major Player Join Date: Oct 2003 Location: Southern Cal-ee-for-Ni-ya Posts: 608	Just some clarification: the lens I was using was originally for aerial photography. I believe the sharpness and grain imaged was identical to a 35mm still camera lens. Unfortunately , the hotspot was also pretty much the same. Even though the rear lens element was 4 in. in diameter! <<<-- Originally posted by Rokta Bija : Les, When you used the medium format lens on your static test, how did the image compare to the 35mm lens as far as sharpness and grain? Was your GG large enough to be able to use the full size of the medium format image? -->>>